A photovoltaic element has attracted attention as a clean energy source which does not discharge CO2, and the photovoltaic element has come into wide use. Currently, the photovoltaic element most widespread is a single-junction solar cell which is manufactured by using silicon. However, in a silicon solar cell in the related art, it is difficult to photoelectrically convert light in a long wavelength region in the solar spectrum, and a large amount of solar energy is not used.
A solar cell having a superlattice structure in terms of efficiently using solar energy unused has attracted attention (for example, see NPL 1). The superlattice structure corresponds to a structure in which quantum layers (quantum dot layers or quantum well layers) having substantially a thickness of several nm to 100 nm and barrier layers are alternately stacked. In the superlattice structure, an intermediate energy band can be formed between an upper end of a valence band of the barrier layer and a lower end of a conduction band of the barrier layer.
A photoelectric conversion element which has a superlattice structure and in which an intermediate energy band is formed has a band diagram as illustrated in FIG. 15, for example. An electron in the valence band is photoexcited to an intermediate energy band 125 by incident light, and an electron 122 in the intermediate energy band 125 is photoexcited to a conduction band 130 by incident light. An internal electric field causes an electron 128 in the conduction band to move to an n-type semiconductor layer, and thus light energy can be converted into electrical energy.
The intermediate energy band 125 has a potential gradient which is formed by the internal electric field. However, the gradient is small in many cases. The electron 122 in the intermediate energy band 125 is subjected to any of the following processes: (1) moving in the intermediate energy band 125; (2) optically transitioning to the conduction band; and (3) recombining with a hole 123 in the valence band.